The rider for 60014: Goblin Commando on Dog is a separate mini and could work. Also, the GW Goblin Wolf Riders have separate riders (plastic, FWIW). Gamezone Miniatures have some goblin wolf riders, but I do not know if the riders are separate from the mounts.

He has a good arch to his legs. I've also mounted a couple of Reaper beastmen onto the axebeaks. You either want long legs set far apart, or you're just going to have to cut a lot of metal away from between. What's nice about the axebeaks is that they have this feathered hump you can build up with greenstuff to get under the rider's crotch.

These goblins from Privateer have a good, wide stance and can be purchased individually from their parts department:

Me again. Otherworld has a whole bunch of goblin riders. I've seen the greens and the riders are definitely separate scultps/sculptors from the wolves. Whether they will sell them to you separately, I don't know. But they're decent gobbos and ready for mounting.